Photoinduced electron transfer from quantum dots to TiO₂: Elucidating the involvement of excitonic and surface states

Colloidal semiconductor quantum dots (QDs) exhibit excitonic and surface states, both of which may participate in charge-transfer processes relevant to solar energy conversion. To explore this inherent complexity of the charge-transfer mechanisms of QDs, we used steady-state and time-resolved emission measurements to characterize excited-state electron transfer (ET) from core-only CdSe QDs and core/shell CdSe/ZnS QDs to TiO₂ nanoparticles (NPs). Core-only QDs transferred electrons from both excitonic and surface states to TiO₂ with rate constants of ET (k_et) of approximately (1-3) × 10⁸ s^-1 and (4-7) × 10⁷ s^-1, respectively. Efficiencies of ET (η_et) from excitonic and surface states were approximately 71-82% and 64-76%, respectively. Thus, trapping of electrons lowered their potential energy but did not greatly affect the efficiency of their transfer to TiO₂. Photogenerated holes were transferred from core-only CdSe QDs to adsorbed 3-mercaptopropionic acid (MPA), which linked the QDs to TiO₂. We characterized core/shell CdSe/ZnS QDs as alternatives to core-only QDs. The ZnS shell eliminated the undesirable trapping of electrons and transfer of photogenerated holes to MPA. We measured k_et of approximately (1-3) × 10⁸ s^-1 and η_et of approximately 66-85% for ET from excitonic states of core/shell CdSe/ZnS QDs to TiO₂ NPs. The insensitivity of k_et to the presence of the ZnS shell may have arisen from increased cross-linking of core/shell QDs to TiO₂. Our results highlight the involvement of surface states in excited-state ET processes of core-only QDs and, for the heterostructures reported here, the improved performance of core/shell CdSe/ZnS QDs relative to core-only CdSe QDs.